1. Field of the Invention
The present invention relates generally to optical sensors, and in particular, to a system and method for sensing intraocular pressure.
2. Description of Related Art
Glaucoma is a leading cause of blindness, affecting an estimated four million Americans and seventy million individuals globally. As glaucoma typically affects the elderly, the aging demographic trends indicate that this disease will continue to be an ever-increasing socioeconomic burden to society. Elevated intraocular pressure (“IOP”) is a major risk factor for glaucoma, and IOP monitoring is the single most important clinical management tool.
Despite the pervasive use of IOP readings for disease monitoring and the clinically proven importance of the aggressive lowering of IOP, current clinical management is primarily based on only periodic snapshots of IOP in the doctor's office obtained every few months. The inability of patients to easily monitor their own IOPs at different times of the day or during various daily activities hinders the comprehensive understanding of the IOP profile of individual patients and the possibility of custom-tailored IOP control.
In addition to its use as a patient monitoring parameter, IOP is also the standard readout used in glaucoma research. However, achieving an acceptable level of accuracy and repeatability in animal IOP measurements requires anesthesia and extreme care. Conducting such time-consuming measurements in large populations of animals is a major hurdle in glaucoma drug discovery.
The need for better IOP monitoring in clinical ophthalmology and in disease research has been widely appreciated. Existing measurement techniques in clinical use measure IOP indirectly. Current IOP measurements involve a form of contact or noncontact applanation tonometry. However, both modalities have difficulties in providing reliable and repeatable readouts of actual IOP values inside the eye. All tonometers produce indirect IOP readings by deforming the ocular globe and correlating this deformation to the pressure within the eye. Their readouts are heavily influenced by the corneal curvature and thickness, or corneal mechanical properties that vary due to co-existing ocular pathologies. For example, patients who have received laser photorefractive keratectomy have thinner corneas in the treated eyes and consistently show lower IOP when measured using tonometry techniques.
Tonometry currently requires specialized equipment operated by an ophthalmologist, optometrist, or skilled technician. Hence, IOP measurements are made typically in a doctor's office about two to four times per year. Since studies show that IOP varies widely throughout the day, quarterly measurements are poor representations of a patient's actual IOP profile.
A number of efforts have also been made to develop MEMS-based implantable IOP sensors with telemetric sensing. Unfortunately, the operating principle of this device puts a limit on the miniaturization of the sensor. Either the size of the sensor has to become large to achieve a longer transmission distance, or small devices lead to extremely short readout distances limit the practical use of the device. For example, to read IOP at a 2 centimeter distance, the IOP sensor has to be at least around 3 millimeters in size, which is too large in terms of patient acceptance and interference with ocular function.
The identification of new therapeutic compounds for glaucoma treatment utilizes IOP reduction in research animals as a screening parameter. Unfortunately, IOP measurements in animals using tonometry require anesthesia and extreme care for repeatability. Previously developed MEMS-based sensors are too large for use in rodent models, which make up more than 90% of the animal species used in glaucoma research. For example, these implants may range in size from 1-3 mm and are difficult to use in rodents that have corneal diameters of about 3.5 mm.